Encephalitic arthropod-borne viruses are emerging as a major cause of death and disease worldwide. Understanding the mechanisms involved in severe and fatal encephalitic infections is critical in developing effective interventions. Sindbis virus (SV), the prototypic alphavirus, is related to Venezuelan, eastern, and western equine encephalitis viruses, which cause seasonal outbreaks of encephalitis in the Americas. SV causes mild disease in humans, but has a neuronal tropism and causes encephalomyelitis in mice. The virulence of SV is age-dependent and virus-strain dependent. Neuroadapted Sindbis virus (NSV) is a virulent strain of SV that causes fatal encephalomyelitis in mice of all ages. Viral clearance correlates with the onset of paralysis suggesting that the immune response is causing pathology, not viral replication. Inhibition of the immune response by treating with pharmaceuticals or by using immunodeficient mouse models leads to protection during infection, further supporting the notion of immunopathology. Glutamate excitotoxicity is also implicated in fatal disease. Interleukin-10 (IL-10) is an anti-inflammatory molecule and a key regulator of both the innate and adaptive arms of the immune response. Its classic effects are to reduce T cell activation by inhibiting expression of MHC class II and co-stimulatory molecules. This dampens the immune response by reducing the production of pro-inflammatory cytokines and chemokines. IL-10 can also directly protect neurons from apoptosis in models of glutamate excitotoxicity and other stress conditions. The proposed research is aimed at determining the role IL-10 has in the central nervous system during fatal viral encephalitis. First, the critical source of IL-10 in the central nervous system (CNS) mediating mortality during NSV infection will be determined. IL-10 reporter mice will be used to evaluate which cells are producing IL-10 in the CNS. The source of IL-10 necessary for mediating protection will be determined by using mice that are deficient in either myeloid- or lymphoid-derived IL-10. Second, the role of IL-10 on regulating TH17 mediated immunopathology will be examined. IL-10 deficient (IL-10 -/-) mice will be used to determine the extent of inflammation and production of neurotoxic cytokines that occurs without IL-10 regulation. The amount of TH17 infiltrates and its products, IL-17, IL-23, and IL-21, will be quantified and survival in the absence of IL-10 and TH17 cells will be determined. Third, the role of IL-10 as a neuroprotective agent will be determined by exogenous administration to infected areas using NSV as a vector for IL-10 (dsNSV:IL-10) and evaluating its effects on morbidity and survival in IL-10 -/- and wild-type mice, determining its effects on neuronal apoptosis in vitro and in vivo, and examining its role in glutamate excitotoxicity. Determining the role of IL-10 in mediating fatal viral encephalomyelitis pathology will increase our understanding of the mechanisms responsible for severe and fatal infections and provide a basis for therapeutic development.